Is books a collective noun or common noun?

The noun 'books' is a common noun, the plural form for the singular noun 'book', a word for a thing (or things).

A collective noun is a word used to group people or things taken together as one whole. The collective nouns for 'books' are a library of books, a compendium of books, a pile of books.

However, any noun is a collective noun when used for that function; for example a book of stamps or books of stamps, a book of poems or books of poems, etc.